Is the urge to breath a reliable sign at depth? Daltons law, P02 and PCO2...

Csm · Oct 23, 2019

Hi all,

Long time lurker first time poster.

My name's Chris, I'm a spearo from the UK living in France.

I've been in the sport for a couple of years and had a question about the reliability of urge to breathe at depth. My current max depth is 18 meters and I'm wondering how safe it is to stay on the bottom until I get an urge to breathe at such depths?

I understand that Dalton's law means that the increase in partial pressure at depth makes O2 more easily transfer from lungs into the blood even at low O2 levels - something which reverses as we ascend.

I also understand that CO2 doesn't follow the same pattern of diffusion with depth - it's largely unaffected.

My questions then are (based on a dive profile that has ZERO hyperventilation):

1. Does this mean that we are burning through O2 much quicker at depth than we would at the surface?
2. If so, does this completely throw off the reliability of the CO2 and O2 relationship? Is the urge to breathe therefore no longer a valid signal of O2 levels at depth as it would be at the surface in a no pre-dive hyperventilation profile?
3. If this is the case, what can we use as a signal to ascend before O2 levels drop below a level that will kill us on surfacing?

I've had a look through a number of posts on the forum but I cannot find a straight answer - if anyone can answer these questions directly or point me in the direction of a post that can help then I'd be really grateful!

Thanks

cdavis · Oct 23, 2019

Csm said:
Hi all,

Long time lurker first time poster.

My name's Chris, I'm a spearo from the UK living in France.

I've been in the sport for a couple of years and had a question about the reliability of urge to breathe at depth. My current max depth is 18 meters and I'm wondering how safe it is to stay on the bottom until I get an urge to breathe at such depths?

I understand that Dalton's law means that the increase in partial pressure at depth makes O2 more easily transfer from lungs into the blood even at low O2 levels - something which reverses as we ascend.

I also understand that CO2 doesn't follow the same pattern of diffusion with depth - it's largely unaffected.

My questions then are (based on a dive profile that has ZERO hyperventilation):

1. Does this mean that we are burning through O2 much quicker at depth than we would at the surface?
2. If so, does this completely throw off the reliability of the CO2 and O2 relationship? Is the urge to breathe therefore no longer a valid signal of O2 levels at depth as it would be at the surface in a no pre-dive hyperventilation profile?
3. If this is the case, what can we use as a signal to ascend before O2 levels drop below a level that will kill us on surfacing?

I've had a look through a number of posts on the forum but I cannot find a straight answer - if anyone can answer these questions directly or point me in the direction of a post that can help then I'd be really grateful!

Thanks

Urge to breath is not a good marker for when to come up. Its way to individual and changes with the body's sensitivity to co2(which goes down as breath holds increase) and the strength of blood shift.

To question 1. other things being equal, 02 burn at depth is slower because of blood shift, which allows the bodies skeletal muscles to go into anerobic mode when there is still plenty of 02 in the core. Blood shift strength is VERY individual and varies between dives.

I've played safety diver for an open water(not line diving) BO when the diver and no significant urge to breathe . On the other hand, some divers have a strong, early urge to breathe(poor co2 tolerance), like me. I normally stay on the bottom well beyond the first urge to breathe.

Csm · Oct 23, 2019

cdavis said:
Urge to breath is not a good marker for when to come up. Its way to individual and changes with the body's sensitivity to co2(which goes down as breath holds increase) and the strength of blood shift.

To question 1. other things being equal, 02 burn at depth is slower because of blood shift, which allows the bodies skeletal muscles to go into anerobic mode when there is still plenty of 02 in the core. Blood shift strength is VERY individual and varies between dives.

I've played safety diver for an open water(not line diving) BO when the diver and no significant urge to breathe . On the other hand, some divers have a strong, early urge to breathe(poor co2 tolerance), like me. I normally stay on the bottom well beyond the first urge to breathe.

Thanks for your reply.

Interestingly I've just come across this post (of course this happens after I post my question!) by Trux on this thread (https://forums.deeperblue.com/threads/some-iscience-on-swb.78976/)

He says the following:

"You lose consciousness (black out) at venous partial pressure (PvO2) of 17-19 mmHg. The venous pressure is lower than arterial pressure (PaO2), hence the PaO2 limit is roughly 20-35 mmHg. The alveolar pressure (PAO2) would need to be slightly higher to assure successful diffusion of oxygen through the alveolar wall - so the critical limit for the pressure in lungs may be as high as 40 mmHg.

On surface, the PaO2 is around 100 mmHg (without hyperventilation). Now, you are right that at depth, the pressure grows, so at 10m it would be double of that, at 30m four times higher.

When you are 30m deep the gas exchange works indeed better due to the higher PAO2, so you can stay down consuming O2 until it drops for example to 80 mmHg, and still have no hypoxic signs (I deliberately do not speak about CO2 yet). When you start surfacing - at 10m the 80 mmHg will drop to half due to the ambient pressure change. That makes 40 mmHg and it is already pretty close to the critical limit. By ascending higher, the PAO2 drops to 20 mmHg (on the surface), which is already deep in the blackout zone.

Note: I ignored the continuing O2 consumption during the ascent.

Fortunately for us, the above mechanism is greatly controlled by CO2. When in depth, the alveolar partial pressure of CO2 also grows proportionally, hence the CO2 stops diffusing into lungs, and cumulates in blood and tissue instead. It increases blood acidity, shifts the gas dissociation curve - helps discharging O2 better in the tissue, but prevents the efficiency of binding O2 remaining in lungs. It also causes lower blood pH, which then controls your urge to breath. So normally, in depth, due to the reduced diffusion of CO2 into lungs, you start feeling the urge to surface and breath earlier than on surface. The problem is if you hyperventilated - the CO2 level will be lower, the pH higher, and the urge to breath may come too late"

This would seem to suggest then that, as long as we avoid hyperventilation, the urge to breath should be relatively reliable even at depth.

Though this post was written in 2008 - has understanding changed since then?

cdavis · Oct 24, 2019

Csm said:
Thanks for your reply.

Interestingly I've just come across this post (of course this happens after I post my question!) by Trux on this thread (https://forums.deeperblue.com/threads/some-iscience-on-swb.78976/)

He says the following:

This would seem to suggest then that, as long as we avoid hyperventilation, the urge to breath should be relatively reliable even at depth.

Though this post was written in 2008 - has understanding changed since then?

Good for you, looking back at old threads. There is a huge amount of knowledge there. Trux was seldom wrong on that kind of detail, but individual variation makes a general rule like urge to breathe not very useful.

Sorandril · Oct 30, 2020

The understanding now appears to be that we are suicidal and mentally damaged and no one should ever do this.

I wish this weren’t so. But sad facts are sad facts. I’m willing to admit I’m miswired and shouldnt reproduce: most others of my kind have drunk the liberal kool ade.

Is the urge to breath a reliable sign at depth? Daltons law, P02 and PCO2...

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Is the urge to breath a reliable sign at depth? Daltons law, P02 and PCO2...

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